f# - MIX seqbuilder [CustomOperation] attribute method AND vanilla yield IN a single seq expression -

this code runs fine except if uncomment last line in custom seq expression :

type t (i: int) =      member x.i =     override x.tostring() =         sprintf "t %a " x.i  type tbuilder() =     member x.yield (()) = seq.empty      [<customoperation("test")>]     member x.test1 (source : seq<_>, i: int) : seq<t> =         printfn "calling test1 i= %d" |> ignore         seq { yield! source               yield t(i) }  let t = tbuilder()  let mytest =      t {         test 42         test 43 //        yield t(44)  // if uncommented, not compile     } 

if yield t(44) line uncommented, compiler error :

error   control construct may used if computation expression builder defines 'for' method. 

my question : there way mix

• my [customoperation] test (from method test1) yields t objects

with

• a vanilla yield, example yield t(44) or other seq related syntax

inside unique seq expression without defining 'for' method ?

reference : dsl in action f# (chapter 7) anh-dung phan on github. thanks.

short answer: no. if change operators preserve variable bindings (via maintainsvariablespace=true or maintainsvariablespaceusingbind=true arguments [<customoperator>] attribute constructor) won't need for you'll need bind instead.

what expect computation expression you've written mean? if @ how f# spec specifies translation computation expressions, of form

bldr {   op1 x   op2 y   yield z } 

will turn

bldr.for(bldr.op2(bldr.op1(bldr.yield(), x), y), fun () -> b.yield(z)) 

so need for method , yield method needs different; @ least needs able take arguments of arbitrary types (e.g. in above example needs work on argument of type unit , on argument of whatever type value z has).